If you don't already pore over your credit card and bank account activity statements, you should really start. "Grey charges," or small fees, subscriptions, renewals, and other charges that many of us don't notice, can add up to hundreds of dollars each year, and are easily removed or reversed if you catch them.

Ginger, founder of Girls Just Wanna Have Funds, explains in a piece at US News that grey charges can start out small, and may even look legitimate. A month's subscription to a service you previously canceled get charged anyway and you don't notice it, for example, or a "free" product you signed up for goes premium after a certain number of months—something buried in its fine print at signup. Even if they're small, they can add up:

Believe it or not, they can cost the average consumer $215 to $350 per year. On a monthly basis it may not seem like much, but over time, this adds up – especially when it happens across several credit card accounts.

In 2012 alone, grey charges cost U.S. credit and debit cardholders $14.3 billion, according to a study by the personal finance security service BillGuard. As you can see, this is a lucrative business practice at your expense. Seems sleazy, right? Without a doubt the practice is less than scrupulous, but in most cases, it is legal. Therefore it is up to you to become vigilant when signing up for a subscription or completing a purchase online.

This is why it's even more important to make sure you take time to review your statements when you get them, or sign up for a service like Mint (which we still love) or Billguard (which we've mentioned before), the former of which makes it easy to see every dollar leaving each of your accounts, and the latter which can automatically protect you from unwanted charges. We've talked about how some companies will just sign you up for extra "services" when you renew a contract like a cell phone plan. This is just another reason to be vigilant, and call immediately to refuse an unwanted charge or get it reversed.